DETERMINATION OF THE POINT-SPREAD FUNCTION FOR THE FERMI LARGE AREA TELESCOPE FROM ON-ORBIT DATA AND LIMITS ON PAIR HALOS OF ACTIVE GALACTIC NUCLEI

• Published in: The Astrophysical journal Vol. 765; no. 1; pp. 1 - 19
• Main Authors: Ackermann, M, Ajello, M, ALLAFORT, A, Asano, K, Atwood, W B, Baldini, L, Ballet, J, Barbiellini, G, BASTIERI, D, Bechtol, K
• Format: Journal Article
• Language: English
• Published: United States American Astronomical Society 01.03.2013; Institute of Physics (IOP)
• Subjects: Active galactic nuclei; ASTRONOMY AND ASTROPHYSICS; Astrophysics; Asymptotic properties; Computer simulation; galaxies: active; galaxies: halos; gamma rays: galaxies; High Energy Astrophysical Phenomena; instrumentation: detectors; intergalactic medium; Mathematical models; Photons; Physics; Sciences of the Universe; Space environment; Spacecraft; Telescopes; intergalactic medium; instrumentation: detectors; gamma rays: galaxies; galaxies: active; galaxies: halos
• ISSN: 0004-637X; 1538-4357; 1538-4357
• DOI: 10.1088/0004-637X/765/1/54

The Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope is a pair-conversion telescope designed to detect photons with energies from [asymptotically =]20 MeV to >300 GeV. The pre-launch response functions of the LAT were determined through extensive Monte Carlo simulations and beam tests. The point-spread function (PSF) characterizing the angular distribution of reconstructed photons as a function of energy and geometry in the detector is determined here from two years of on-orbit data by examining the distributions of gamma rays from pulsars and active galactic nuclei (AGNs). Above 3 GeV, the PSF is found to be broader than the pre-launch PSF. We checked for dependence of the PSF on the class of gamma -ray source and observation epoch and found none. We also investigated several possible spatial models for pair-halo emission around BL Lac AGNs. We found no evidence for a component with spatial extension larger than the PSF and set upper limits on the amplitude of halo emission in stacked images of low- and high-redshift BL Lac AGNs and the TeV blazars 1ES0229 + 200 and 1ES0347-121.